The present invention relates to an image forming apparatus which forms an image in a wet developing process using a liquid developer.
A copying apparatus that uses an electrostatic copying process is one example of an image forming apparatus. In this type of copying apparatus, an image on a document to be copied is represented in the form of image information that indicates the optically light and shade portions of the image. An electrostatic latent image is formed on a photosensitive body in accordance with the image information, and the electrostatic latent image is then visualized with toner (i.e., a developer), thus making a copy of the document. As a method for supplying toner to the electrostatic latent image, the use of a liquid developer is known in the art. The liquid toner is a solution in which toner particles are dispersed.
In comparison with an image forming apparatus that utilizes a magnetic brush phenomenon, an image forming apparatus that employs a liquid developer is advantageous in that it can use toner having small-sized particles and thus contributes to improvement of image quality. In addition, the image forming apparatus that employs a liquid developer ensures a high-speed image formation. Moreover, the liquid developer is improved in gradation reproducibility, and a high-definition image which is equal in quality to a printed image can be obtained. It should also be noted that the temperature at which the toner begins to soften is comparatively low, so that a fixing operation of an toner image is easy to perform.
A liquid developer is made up of toner particles and a carrier solution. This being so, there is room for improvement in regard to the present method in which the liquid developer is supplied to the electrostatic latent image, and the present method in which residual liquid toner (i.e., the carrier solution) remaining on the image bearer (photosensitive body) after the developing process is removed.
To remove the carrier solution that remains on the photosensitive drum after the developing process, a variety of methods have been proposed to date. One of them is a system which employs a non-contact metal roller, another is a nozzle suction system which utilizes air suction.
U.S. Pat. No. 5,576,815 discloses a squeeze system in which the carrier solution is squeezed out of a photosensitive body by means of an elastic roller. U.S. Pat. No. 5,335,054 discloses a setting mechanism wherein a toner image is set or fixed on the surface of a photosensitive drum by bringing an elastic roller formed of an elastic polymer into contact with the photosensitive drum.
If the residual carrier solution that is left on the photosensitive body is not removed after the developing process, it will form a layer having a thickness of 10 .mu.m to 100 .mu.m, though this thickness varies depending upon the type of an image forming apparatus employed and upon the process speed or other factors.
Unless the carrier solution is squeezed out of the photosensitive body, the carrier solution (liquid developer) stored in the developing device will be used up in a comparatively short period of time. In addition, the residual toner will become a cause of a toner image being distorted in the transfer process, resulting in a defective image. If the residual carrier solution is not completely squeezed and evaporates into the interior of the apparatus, it may give adverse effects on the movable portion, the metal portions, the electric circuit portions of the apparatus.
In the system employing a non-contact metal roller, a squeeze roller is kept arranged at a position about 50 .mu.m away from the surface of the photosensitive body, and that squeeze roller is rotated in the same direction as the photosensitive body (which means that the outer circumferential surface of the photosensitive body and that of the squeeze roller are moved in the opposite directions in the region where they face to each other). In spite of the adoption of this system, however, it is likely that the residual carrier solution will form a layer of 2 .mu.m to 4 .mu.m on the photosensitive body.
The nozzle suction system which employs an air nozzle is expected to remove a residual carrier solution in a reliable manner when it is applied to an apparatus whose process speed is low. However, the apparatus incorporating the nozzle suction system is inevitably large since the nozzle suction system requires a suction apparatus comprising a vacuum pump as a suction source. If the nozzle suction system is applied to an apparatus whose process speed is high, the suction apparatus must have improved performance corresponding to the high process speed. Such a suction apparatus is not suitable for practical use since it gives rise to an increase in the power required and is noisy.
In the system employing elastic roller, the surface characteristic of the roller is a factor that plays an important role in the removal of the residual carrier solution. To be more specific, the surface of the roller is elastic; it is low in the degree of smoothness. If the surface roughness is represented as the value of Rz, it is in the range of 10 to 20 .mu.m. As long as the roller has such a surface roughness, a satisfactory squeezing effect cannot be obtained.